Avaliação experimental de um sistema de refrigeração em cascata utilizando o dióxido de carbono como fluido refrigerante

Abstract

This experimental study evaluates the performance of a cascade system in subcritical operation using the pair R744/R134a as an alternative to conventional direct expansion systems installed in supermarkets. The experimental apparatus is, strategically, separated into two cycles, the low temperature cycle (LT), which is composed of a reciprocating semi-hermetic compressor with variable speed control and an electronic expansion valve, that promotes direct evaporation of the CO2 inside a cold chamber to maintain the internal air temperature stable. The high temperature cycle (HT) is, originally, comprised of another reciprocating semi-hermetic compressor for R134a, a thermostatic expansion valve, and an air-cooled condenser. A plate heat exchanger, which is at the same time, the condenser for the R744 and the evaporator for the HT cycle, completes the bench setup. In the methodology, the limits of the original cascade system, R744/R134a, have been explored by manipulating two operational parameters: the degree of superheat (LT), 5 - 20 ºC, and the R744 compressor operating frequency, 40 - 65 Hz. In the second step was performed drop-in of R134a by R438A. And, in sequence, R404A was introduced into the HT cycle. Through the results, it was found that the pair R744/R438A attended the best drop-in condition. The R744/R404A system also achieved the same R134a operating ranges. In addition, the estimative of the environmental impacts of each pair was calculated using the TEWI method, the total equivalent warming impact. The pair R744/R134a had the lowest impacts, while the R744/R404A system promoted the worst damage to the environment.